Indebtedness (Summary of Book Value and Changes in Fair Value of Debt) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
Jun. 30, 2012	Dec. 31, 2011
Book value	$ 4,036,643	$ 3,809,589
Fair value	4,405,196	4,117,230
Issuances and assumptions	332,046
Payments/Payoffs	(103,777)
Adjustments to fair value	59,697
Fixed Rate Secured Debt [Member]
Book value	1,081,845	1,167,188
Fair value	1,176,155	1,256,331
Issuances and assumptions	18,741
Payments/Payoffs	(102,869)
Adjustments to fair value	3,952
Variable Rate Secured Debt [Member]
Book value	19,350	6,045
Fair value	19,848	6,045
Issuances and assumptions	13,305
Payments/Payoffs	0
Adjustments to fair value	498
Unsecured Debt [Member]
Book value	2,915,155	2,616,063
Fair value	3,188,913	2,834,610
Issuances and assumptions	300,000
Payments/Payoffs	(908)
Adjustments to fair value	55,211
Unsecured Lines Of Credit [Member]
Book value	20,293	20,293
Fair value	20,280	20,244
Issuances and assumptions	0
Payments/Payoffs	0
Adjustments to fair value	$ 36

Issues that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

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- Definition

The increase (decrease) of liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period. Such increase (decrease) may be comprised of changes in fair value (gains or losses) whether realized or unrealized that have been included in earnings, changed net asset value, or other comprehensive income (a separate component of shareholders' equity); purchases, sales, issuances, or settlements of assets; and net transfers in to or out of this category within the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c(1), c(2), c(3)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Settlements that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

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This element represents a liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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Including current and noncurrent portions, aggregate carrying amount of long-term borrowings as of the balance sheet date. May include notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, which had initial maturities beyond one year or beyond the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any.

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-Name Regulation S-X (SX)

-Number 210

-Article 7

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